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Blood Glucose Tests

Blood Glucose Tests

Alternate Names

  • blood sugar


Glucose is the most basic type of sugar in the body. The body forms glucose when it breaks foods that a person eats down into a useable form of energy. Glucose is a main energy source for the body.

Who is a candidate for the test?

A doctor may order a blood sugar test to evaluate the amount of glucose in the blood when he or she suspects a person has diabetes. These tests may also be done to rule out other causes of high or low blood glucose.

How is the test performed?

Here's how some of the tests may be done to check blood glucose levels:
  • A fasting blood sugar test is the preferred method to diagnose diabetes and rule out other conditions. This test is done after a person has had nothing to eat or drink except water for at least 8 hours, usually overnight, so the test can be done in the morning.
  • Normal fasting blood sugar levels are less than 100 mg/dL (milligrams per deciliter). Fasting blood sugar levels of more than 126 mg/dL on two or more tests done on different days usually indicate diabetes. Levels between 100 and 126 indicate a condition known as pre-diabetes.
  • An HbA1c, also known as glycosylated hemoglobin, measures the average blood glucose over the past 3 months. It is a good measure of long-term blood glucose control. This test is generally used to assess how well therapy is working in a person with diabetes.
  • An oral glucose tolerance test requires a person to drink a premeasured amount of a glucose drink. Then two hours later, a blood glucose measurement is done. Healthy glucose levels with this test are less than 140 mg/dL. If the blood glucose is greater than 200 mg/dL, then another test is done on a different day to confirm whether the person has diabetes or not.
  • A random blood sugar test is done shortly after a person has eaten or had something to drink. A level of 200 mg/dL or higher may indicate diabetes. Usually if a level is above 200 mg/dL, a fasting blood sugar test or oral glucose tolerance test is done to confirm the diagnosis of diabetes.
  • A self-monitoring of blood glucose, also called SMBG or home blood glucose monitoring, permits an individual with diabetes to keep a record of blood glucose readings to follow changes in the levels throughout the day. The information can be useful to the healthcare professional in deciding if changes need to be made to the person's diabetes treatment plan.
For most blood sugar tests, a blood sample is taken from a vein in the arm. First, the skin over the vein is cleaned with an antiseptic. A thin strip of rubber called a tourniquet is wrapped around the upper arm to enlarge the veins.
A small needle is gently inserted into the enlarged vein. Then blood is collected for testing in the laboratory. After the tourniquet is removed, a cotton ball is held over the needle site until bleeding stops. The laboratory then tests the blood sample.
The home blood glucose monitoring system, on the other hand, uses a small drop of blood from a person's finger. The drop is put onto a special strip of paper, which is then inserted into the blood glucose monitor machine. The monitor measures and displays the blood glucose reading. This test can monitor blood glucose with reasonable accuracy if the person follows the directions carefully.

What do the test results mean?

The normal blood glucose level varies depending on:
  • which test was performed
  • whether a person was fasting before the test
  • whether any special dietary or glucose substances were given during testing
Increased levels of blood glucose, a condition known as hyperglycemia, may be caused by the following:
  • acromegaly, a condition that causes elongation of the bones of the limbs and head
  • Cushing syndrome, a condition in which the level of the hormone cortisol is too high and causes fatigue, weakness, protein loss, swelling, and diabetes mellitus, which is also called DM
  • diabetes mellitus
  • diuretics, also known as water pills
  • gestational diabetes (diabetes that develops during pregnancy)
  • inadequate therapy for diabetes mellitus
  • infection in the pancreas, known as pancreatitis
  • kidney failure, such as chronic renal failure
  • liver disease, such as cirrhosis
  • pheochromocytoma, a non-malignant tumor that causes an increase in certain chemicals responsible for high blood pressure
  • steroid medicines, such as prednisone
  • stress response, including infection, severe burns, or surgery
Decreased levels of blood glucose, a condition known as hypoglycemia, may be caused by the following:
  • Addison disease, a condition in which there is a decreased amount of the adrenocorticol hormone
  • blood loss
  • extensive liver disease
  • hypopituitarism, a condition in which the pituitary gland does not release enough hormone
  • hypothyroidism, a condition in which too little thyroid hormone is present in the blood
  • insulin overdose
  • insulinoma, which is a tumor in the pancreas that causes too much insulin to be produced
  • malabsorption, or inadequate absorption of nutrients from the stomach or intestines
  • starvation


Diagnostic and Laboratory Test, Kathleen Pagana and Timothy Pagana, 1998 [hyperLink url="" linkTitle=""][/hyperLink]

American Diabetes Association: Clinical Practice Recommendations 2000. Diabetes Care 2000;23(suppl):S1-116.

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